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Degradation Kinetics of Model Hyperbranched Chains with UniformSubchains and Controlled Locations of Cleavable Disulfide LinkagesLianwei Li,*,† Xu Wang,† Jinxian Yang,† Xiaodong Ye,*,† and Chi Wu†,‡

†Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics, University of Science andTechnology of China, Hefei, Anhui 230026, China‡Department of Chemistry, The Chinese University of Hong Kong, Shatin N. T., Hong Kong

ABSTRACT: We developed a strategy to make modelhyperbranched structure with uniform subchains and con-trolled locations of cleavable linkages. First, a novel seesaw-type tetrafunctional initiator with one alkyne, one disulfidelinkage, and two bromine groups (≡−S−S−(Br)2) wasprepared. Using such an initiator, an AB2-type macromonomer(azide∼∼alkyne∼∼azide) with one disulfide linkage at itscenter was prepared via successive atom transfer radicalpolymerization (ATRP) and azidation substitution reaction,where ∼∼ represents polystyrene chains. Further interchain“clicking” coupling between the azide and alkyne groups on themacromonomers led to model hyperbranched polystyreneswith uniform subchains and controllablly located cleavabledisulfide linkages. The 1H nuclear magnetic resonance spectra, Fourier transform infrared spectroscopy, and size exclusionchromatography with a multiangle laser light scattering detector confirmed the designed degradable hyperbranched structure.Armed with this novel sample, we studied its dithiothreitol (DTT)-induced degradation in various organic solvents by acombination of static and dynamic LLS. We found that the cleavage of disulfide bonds contains a fast and a slow process. The fastone reflects the degradation of disulfide bonds on the chain periphery; while the slow one involves those inside. Both the fast andslow degradation reaction rate constants (Kfast and Kslow) are a linear function of the initial DTT concentration ([DTT]0), but therelative contribution of the two processes is mainly governed by the hyperbranched chain structure, nearly independent of[DTT]0.

■ INTRODUCTIONIn the past two decades, various nonideal hyperbranchedstructures with uncontrollable branching subchains have beenextensively prepared and used for the structure−propertyinvestigation.1−7 This is due to the fact that the preparation ofan ideal model hyperbranched structure with uniform andadjustable subchains between any two neighboring branchingpoints is rather difficult because of synthetic methodologylimitation,8,9 which seriously hinders the relevant structure−property study.10,11 Recently, using a novel seesaw-typemacromonomer B∼∼A∼∼B as the precursor in self-poly-condensation reaction, where the reactive A and the two Bgroups are located at the chain center and the two chain ends,respectively,12−14 we have successfully prepared model hyper-branched chains. Moreover, we have experimentally, for the f irsttime, established the scaling laws between their sizes (R),intrinsic viscosities ([η]), and molar masses (M);13,15 foundhow these chains pass through a small cylindrical nanoporeunder an elongational flow field;16,17 and prepared amphiphilichyperbranched block copolymer chains and studied theirinterchain and intrachain association in dilute and semidilutesolutions.18

On the other hand, it is well-known that disulfide bonds canbe cleaved into thiol groups in the presence of various reducingagents,19 such as thiols,20 phosphines,21,22 and zinc dust;23 andthe resulting thiol groups can reversibly re-form disulfide bondsupon oxidation. Therefore, the thiol−disulfide exchangereaction has been widely used in various applications, such asdrug delivery,24 gene transfection,25 and construction of self-healing materials.26,27 Meanwhile, a number of disulfide-functionalized polymer structures have been prepared, namely,linear (diblock24,28−30 and multiblock31,32), cyclic,31 comb-like,33 hyperbranched,25,34−38 and cross-linking39−41 structures.Generally, two main approaches have been used to introduce

disulfide linkage into the polymer backbone: (1) homopolyme-rization of star precursors with thiol ends (Scheme 1A)31,37 or(2) copolymerization of star precursors with disulfide-function-alized linear small molecules (Scheme 1B).25,34−36 However,the distributions of the subchain length and disulfide bondlocation of hyperbranched polymers prepared by these twoapproaches will be only partly controllable (star precur-

Received: November 28, 2013Revised: December 27, 2013Published: January 6, 2014

Article

pubs.acs.org/Macromolecules

© 2014 American Chemical Society 650 dx.doi.org/10.1021/ma402438m | Macromolecules 2014, 47, 650−658

pubs.acs.org/Macromolecules

sors).25,33,34,36,42,43 To the best of our knowledge, it remains achallenge to prepare model hyperbranched polymers withuniform subchains and controlled locations of cleavabledisulfide linkages simultaneously.In the current study, applying our seesaw-type macro-

monomer stragtegy, we have successfully synthesized modelhyperbranched polystyrene chains with uniform subchains andcontrollablly located cleavable disulfide linkages. As shown inScheme 2, seesaw-type macromonomer ≡−S−S−(PS−N3)2

with one disulfide linkage at the chain center was first preparedby atom transfer radical polymerization (ATRP) and azidationsubstitution reaction. Further interchain “clicking” coupling ofmacromonomers ≡−S−S−(PS−N3)2 led to disulfide-function-alized hyperbranched polystyrene chains (Scheme 3). The well-defined structures of macromonomer and hyperbranchedproduct were confirmed by various kinds of methods. Finally,the degradation kinetics of hyperbranched polystyrene chains invarious organic solvents was monitored by a combination ofstatic and dynamic laser light scattering.

■ EXPERIMENTAL SECTIONMaterials. Styrene (Sinopharm, 97%) was purified by first passing

it through a basic alumina column and then distilling under a reducedpressure over calcium hydride (CaH2). Dimethylformamide (DMF,Sinopharm, AR) was first dried with anhydrous magnesium sulfate andthen distilled under a reduced pressure. N,N,N′,N′,N″-Pentamethyl-diethylenetriamine (PMDETA, Aldrich, 99%), tetrahydrofuran (THF),dichloromethane (DCM), and triethylamine (TEA) were distilled overCaH2. DMF, THF, and DCM were bubbled with nitrogen for 5 hbefore used. Dithiothreitol (DTT, Aladdin, 99%) was stored undernitrogen atmosphere at 2−6 °C. All the other reagents fromSinopharm or Aldrich were directly used as received unless otherwisestated.

Characterization. 1H nuclear magnetic resonance (1H NMR)spectra were recorded using a Bruker AV400 spectrometer withtetramethylsilane (TMS) as an internal standard and deuteratedchloroform (CDCl3) as solvent. Fourier transform infrared spectra(FTIR) were recorded using a Bruker VECTOR-22 IR spectrometer,and each spectrum was collected over 64 scans with a spectralresolution of 4 cm−1.

Size Exclusion Chromatography (SEC). The relative number-and weight-average molar masses (Mn,RI and Mw,RI) and the absolutenumber- and weight-average molar masses (Mn.MALLS and Mw,MALLS)were determined at 35 °C by size exclusion chromatography (SEC,Waters 1515) equipped with three Waters Styragel columns (HR2,HR4, and HR6), a refractive index detector (RI, Wyatt WREX-02),and a multiangle laser light scattering detector (MALLS, WyattDAWN EOS). THF was used as the eluent at a flow rate of 1.0 mL/min, and linear narrowly distributed polystyrenes were used asstandards.

Laser Light Scattering (LLS). A commercial LLS spectrometer(ALV/DLS/SLS-5022F) equipped with a multi-τ digital timecorrelator (ALV5000) and a cylindrical 22 mW UNIPHASE He−Ne

Scheme 1. Schematic of Topological Structures of Disulfide-Functionalized Hyperbranched Chains Prepared by Two DifferentApproaches: (A) Homopolymerization of Telechelic Star Precursors with Thiol End Groups and (B) Copolymerization ofDisulfide-Functionalized Small Molecules and Star Chains

Scheme 2. Schematic of Synthesis of Disulfide-Functionalized Seesaw-Type Initiator and LinearPolystyrene Macromonomer ≡−S−S−(PS−N3)2

Scheme 3. Schematic of Synthesis and Degradation ofDisulfide-Functional Hyperbranched Polystyrene ChainsHB−(S−S−PS)n

Macromolecules Article

dx.doi.org/10.1021/ma402438m | Macromolecules 2014, 47, 650−658651

laser (λ0 = 632.8 nm) was used as the light source. In static LLS,44,45

the angular dependence of the absolute excess time-average scatteringintensity, known as the Rayleigh ratio RVV(q), can lead to the weight-average molar mass (Mw), the root-mean-square gyration radius(⟨Rg⟩), and the second virial coefficient A2 by using

≅ + ⟨ ⟩ +⎜ ⎟⎛⎝

⎞⎠

KCR q M

R q A C( )

11

13

2VV

Zw

g2 2

2(1)

where K = 4π2n2(dn/dC)2/(NAλ04) and q = (4πn/λ0) sin(θ/2) with n,

C, dn/dC, NA, and λ0 being the refractive index, the concentration ofthe polymer solution, the specific refractive index increment,Avogadro’s number, and the wavelength of light in a vacuum,respectively. The dn/dC value of polystyrene in THF is 0.186 mL/g.12,13

In dynamic LLS,46 the Laplace inversion of each measuredintensity−intensity time correlation function G(2)(q,t) in the self-beating mode can result in a line-width distribution G(Γ). G(Γ) can beconverted to a translational diffusion coefficient distribution G(D) orfurther to a hydrodynamic radius distribution f(Rh) via the Stokes−Einstein equation, Rh = (kBT/6πη0)/D, where kB, T, and η0 are theBoltzmann constant, the absolute temperature, and the solventviscosity, respectively, by using both the cumulants and CONTINanalysis.Synthesis of Intermediate Compound I. The general procedure

was outlined as follows. 30.0 g of 2,2-bis(hydroxymethyl)propionicacid (223.7 mmol), 41.4 mL of 2,2-dimethoxypropane (DMP) (335.4mmol), and 2.1 g of p-toluenesulfonic acid monohydrate (p-TSA, 11.1mmol) were dissolved in 100 mL of acetone. The mixture was stirredfor 4 h at room temperature before a 3.0 mL mixture of an ammoniaaqueous solution (25%) and ethanol (50/50, v/v)) was added into thereaction mixture to neutralize the catalyst. The solvent was removedby evaporation under a reduced pressure at room temperature. Theresidue was then dissolved in DCM (600 mL) and extracted with twoportions of water (80 mL). The organic phase was dried withanhydrous Na2SO4 and then evaporated to give white powder (33.0 g,84%). Its 1H NMR spectrum in CDCl3 can be found elsewhere.

47

Synthesis of Intermediate Compound II. The general procedurewas outlined as follows. 20.0 g of 2-hydroxyethyl disulfide (130 mmol)and 3.6 g of propargyl bromide (65 mmol) were mixed in 200 mL ofTHF. 3.0 g of NaH powder (80 wt %, 98 mmol) was added into thereaction mixture in three successive batches under nitrogen within 3 hat 0 °C. The mixture was further stirred for another 7 h at roomtemperature before a few drops of water were added to stop thereaction. The mixture was filtered, and the solvent was removed byevaporation under a reduced pressure at 50 °C. The crude product waspurfied by column chromatography to give a pale-yellow clear oil: 6.5 g(52%). Figure 1A shows its 1H NMR spectrum in CDCl3.

1H NMR(CDCl3, ppm): δ 4.20 (d, 2H, CHCCH2O−), 3.90 (t, 2H, CHCCH2OCH2−), 3.81 (t, 2H, −CH2OH), 2.93 (t, 2H, CHCCH2OCH2CH2−), 2.88 (t, 2H, −CH2CH2OH), 2.46 (t, 1H,CHCCH2−), 1.93 (s, 1H, −CH2OH).Synthesis of Intermediate Compound III. The general procedure

was outlined as follows. 5.4 g of compound I (31.3 mmol), 6.0 g ofcompound II (31.3 mmol), and 0.38 g of 4-(dimethylamino)pyridine(DMAP) (3.1 mmol) were dissolved in 100 mL of DCM. 7.74 g ofN,N′-dicyclohexylcarbodiimide (DCC) (37.6 mmol) was dissolved in50 mL of DCM. The two solutions were mixed under nitrogen flow at0 °C within 0.5 h and further stirred overnight at room temperature.The byproduct N,N′-dicyclohexylurea was filtered. The solution wasthen diluted by 100 mL of DCM and extracted with two portions ofwater (150 mL). The organic phase was dried with anhydrous sodiumsulfate. After removing the solvent by evaporation, the crude productwas purified by column chromatography to give a pale-yellow oil: 8.0 g(83%). Figure 1B shows its 1H NMR spectrum in CDCl3.

1H NMR(CDCl3, ppm): δ 4.43 (t, 2H, CHCCH2O−), 4.20 (m, 4H,(CH3)2C(OCH2−)), 3.79 (t, 2H, −CH2OOC−), 3.66 (m, 2H, CHCCH2OCH2−), 2.96 (m, 4H, −CH2SSCH2−), 2.46 (t, 1H, CHCCH2−), 1.40 (m, 6H, (CH3)2C(OCH2−)), 1.21 (s, 3H, CH3−).

Synthesis of Intermediate Compound IV. The general procedurewas outlined as follows. 8.0 g of compound III (25.0 mmol) dissolvedin 100 mL of methanol was added with an excess of activated acidicstyrene cation exchange resin (Amberlite IR-120, aladdin). Themixture was stirred for 4 h at room temperature. After the resin wasfiltered, the solvent was removed by evaporation. The crude productwas purified by column chromatography to give a pale-yellow oil: 7.1 g(89%). Figure 1C shows its 1H NMR spectrum in CDCl3.

1H NMR(CDCl3, ppm): δ 4.45 (t, 2H, CHCCH2O−), 4.20 (t, 2H,−CH2OOC−), 3.88 (d, 2H, CHCCH2OCH2−), 3.75 (m, 4H,HOCH2−), 2.95 (m, 4H, −CH2SSCH2−), 2.46 (t, 1H, CHCCH2−), 1.11 (s, 3H, CH3−).

Synthesis of Compound V, Seesaw-Type Initiator. The generalprocedure was outlined as follows. 6.5 g of compound IV (21.1 mmol)and 2.6 g of TEA (25.3 mmol) were dissolved in 150 mL of DCM.11.6 g of α-bromoisobutyryl bromide (50.6 mmol) dissolved in 50 mLof DCM was added into the reaction mixture under nitrogen flow at 0°C within 0.5 h. The mixture was stirred overnight. After filtration, themixture was diluted with 100 mL of DCM and extracted with 50 mL ofsaturated sodium bicarbonate aqueous solution. The organic phase wasdried with anhydrous sodium sulfate. After removing the solvent byevaporation, the crude product was purified by column chromatog-raphy to give a dark-yellow oil: 9.0 g (71%). Figure 2 shows its 1HNMR spectrum in CDCl3.

1H NMR (CDCl3, ppm): δ 4.43 (m, 4H,(COOCH2)2CCH3−), 4.38 (t, 2H, CHCCH2O−), 4.20 (t, 2H,−CH2OOCSS−), 3.80 (m, 2H, CHCCH2OCH2−), 2.95 (m, 4H,

Figure 1. 1H NMR spectra of the intermediate compounds (II−IV) inCDCl3.



−CH2SSCH2−), 2.48 (t, 1H, CHCCH2−), 1.98 (d, 12H,BrC(CH3)2−), 1.36 (s, 3H, CH3−).Preparation of Macromonomer ≡−S−S−(PS−Br)2 by ATRP. The

general procedure was outlined as follows. A three-necked flaskequipped with a magnetic stirring bar and three rubber septa wascharged with 0.4 g of initiator V (0.66 mmol), 12.1 mL of styrene(105.9 mmol), and 124 μL of PMDETA (0.66 mmol). The flask wasdegassed by three freeze−pump−thaw cycles and then placed in an oilbath thermostated at 90 °C. After ∼2 min, 93.7 mg of CuBr (0.66mmol) was added under nitrogen flow to start the polymerization.Samples were withdrawn at different times during the polymerizationfor SEC and monomer conversion measurements by the weighingmethod. Namely, the conversion was calculated as conversion = m/m0,where m0 and m represent the weights of sample solution before andafter removing styrene monomer by evaporation under a reducedpressure at 50 °C for four cycles; in each cycle, a certain amount ofTHF was added to enhance the extraction of styrene. After a fewhours, the flask was rapidly cooled in liquid nitrogen. The polymermixture was diluted with THF and passed through a short column ofneutral alumina to remove metal salt. The solvent was removed byevaporation, and the residue was dissolved in THF and precipitatedinto an excess of methanol and recovered by filtration. The abovepurification cycle was repeated twice. After drying in a vacuum ovenovernight at 40 °C, macromonomer ≡−S−S−(PS−Br)2 was obtained.Yield: 6.15 g (54%). Mn: 9.4 × 10

3 g/mol and Mw/Mn: 1.14 (by SEC);Mn: 9.0 × 10

3 g/mol (by 1H NMR in Figure 2).Preparation of Macromonomer ≡−S−S−(PS−N3)2 by Azidation.

The general procedure was outlined as follows. A 100 mL round-bottom flask was charged with 5.0 g of ≡−S−S−(PS−Br)2 (0.45mmol), 40 mL of DMF, and 0.29 g of sodium azide (4.5 mmol). Themixture was allowed to stir at room temperature for 24 h undernitrogen flow. After removing DMF under a reduced pressure, theremaining portion was diluted with DCM and passed through aneutral alumina column to remove residual sodium salts. The solventwas removed by evaporation, and the residue was dissolved in THFand precipitated into an excess of methanol. After drying in a vacuumoven overnight at 40 °C, azido-terminated polymer was obtained andnamed as ≡−S−S−(PS−N3)2. Yield: 4.7 g (94%). Mn: 9.3 × 103 g/mol and Mw/Mn: 1.12 (by SEC).Preparation of Degradable Hyperbranched Polymer HB-(S−S−

PS)n by “Click” Chemistry. The general procedure was outlined asfollows. A 25 mL three-necked flask equipped with a magnetic stirringbar and three rubber septa was charged with 4.5 g of macromonomer≡−S−S−(PS−N3)2 (∼0.43 mmol, Mw,macromonomer = 1.04 × 104 g/mol), 83 μL of PMDETA (0.40 mmol), and 15.0 mL of DMF. Theflask was degassed by three freeze−pump−thaw cycles and then placedin a water bath thermostated at 35 °C. After ∼2 min, 57.5 mg of CuBr(0.40 mmol) was added under nitrogen flow to start the polymer-ization. Samples were withdrawn at different times and precipitatedinto a mixture of methanol/water (90/10, v/v) for SEC measure-ments. After 24 h, the flask was cooled in liquid nitrogen. The polymermixture was diluted with THF and passed through a short column ofneutral alumina for the removal of metal salt. The solvent was

removed by evaporation, and the residue was dissolved in THF andprecipitated into an excess of methanol. After drying in a vacuum ovenovernight at 40 °C, hyperbranched polymer HB-(S−S−PS)n wasobtained. The resultant polydispersed HB-(S−S−PS)n was furtherpurified by fractional precipitation in a mixed solution of toluene/methanol to remove most of the low-molar-mass fractions. Finally,hyperbranched HB-(S−S−PS)n fraction with a Mw of ∼3.7 × 105 g/mol (n ≃ 35) was obtained and used in the degradation kinetics study.

DTT-Induced Degradation of HB-(S−S−PS)35 Chains. The DTT-induced degradation of HB-(S−S−PS)35 was conducted in situ insidethe LLS cuvette. Stock DMF solutions of HB-(S−S−PS)35 (1.0 g/L)and DTT (25.0 g/L) were purged by nitrogen to replace oxygen andrespectively clarified with 0.45 μm Millipore PTFE filters to removedust. In a typical experiment, a proper amount of dust-free DTT DMFsolution was rapidly added into 3.0 mL of dust-free HB-(S−S−PS)35DMF solution to start the degradation. Both the scattered intensityand G(2)(q,t) were recorded during the degradation for at least 1000min.

■ RESULTS AND DISCUSSIONAs shown in Scheme 2, disulfide-functional initiator (V) wasprepared through a five-step synthesis. All the intermediatecompounds (II−IV) were carefully purified by columnchromatography, and their high purities were confirmed by1H NMR characterization (Figure 1A−C).Figure 2 shows the 1H NMR spectrum of the tetrafunctional

initiator, and the integral area ratio of peak (a), peak (i), andpeak (d + e) is 1.00/3.10/4.09, in consistency with itstheoretical ratio of 1.00/3.00/4.00. Using this functionalinitiator, seesaw-type polystyrene macromonomer ≡−S−S−(PS−Br)2 was successfully prepared by ATRP, and its typical1H NMR spectrum is shown in Figure 3.

Figures 4−6 show how the monomer conversion, number-average molar mass (Mn), polydispersity index (Mw/Mn), andSEC curves of polystyrene evolved during the ATRP process.As shown in Figure 4, the plot of semilogarithmic of ln(1/(1 −conversion)) versus t presents a linear relation, indicating thepolymerization meets the required criteria of a living system.Figure 5 shows that the experimental Mn increases linearly withthe conversion, and the Mw/Mn rapidly decreases with theconversion and reaches a minimum of ∼1.14 at the end ofpolymerization, indicating the whole ATRP process is well-controlled, which is also reflected in the symmetric andnarrowly distributed elution curves (Figure 6).In a typical ATRP process, the molar ratio of CuBr to

halogen atom on initiator is normally 1.0/1.0, while the ratio in

Figure 2. 1H NMR spectrum of disulfide-functionalized initiator.

Figure 3. 1H NMR spectrum of disulfide-functionalized ≡−S−S−(PS−Br)2.



our study is actually 1.0/0.5. In addition, the monomerconversion was intentionally controlled below 60%, which isbecause both the CuBr concentration and monomer conversionare the key factors to attain polymer chains with high end-group functionalities. In principle, lowering both the CuBrconcentration and monomer conversion can effectively reducethe chain termination and other side reactions.48,49

Further azidation substitution reaction of ≡−S−S−(PS−Br)2by sodium azide in DMF led to macromonomer ≡−S−S−(PS−N3)2. The successful installation of azide groups wasreflected in the appearance of a NNN antisymmetricstretching absorption band near ∼2090 cm−1 in FT-IR spectra(Figure 7). Using this seesaw-type macromonomer ≡−S−S−(PS−N3)2, we are able to prepare degradable hyperbranched

polystyrene chains with uniform subchains and controlledlocations of cleavable disulfide linkages. Scheme 3 schematicallyshows the structures of the prepared hyperbranched chains andits degradation products.The alkyne−azide “click” cycloaddition was used here

because of its high efficiency and low susceptibility to sidereactions.50−52 DMF was used as solvent in the interchain“clicking” of macromonomer ≡−S−S−(PS−N3)2 after theremoval of oxygen at 35 °C with a catalyst of CuBr−PMDETA.Similar to our previous study,12,13 we prefer to use the weight-average polycondensation degree (DPw), instead of thenumber-average polycondensation degree (DPn), to describethe whole interchain “clicking” process because it is the weight-average value that could be measured accurately in SEC-MALLS or stand-alone static LLS, where DP is defined as thenumber of macromonomers, not monomer, chemically coupledtogether inside each hyperbranched polystyrene chain; namely,DPw = Mw,hyperbranched/Mw,macromonomer (Mw,macromonomer 1.04 ×104 g/mol).Figure 8A shows that the two elution curves respectively

from the RI and MALLS detectors are very different. This is

Figure 4. (A) Kinetic plot of semilogarithmic of ln(1/(1 −conversion)) versus reaction time (t) during the ATRP process.Reaction conditions: [styrene]/[initiator]/[CuBr]/[PMDETA] =150/1/1/1; T = 90 °C; bulk.

Figure 5. Monomer conversion dependence of number-average molarmass (Mn) and polydispersity index (Mw/Mn) of macromonomer ≡−S−S−(PS−Br)2 during the ATRP process. Reaction conditions:[styrene]/[initiator]/[CuBr]/[PMDETA] = 150/1/1/1; T = 90 °C;bulk.

Figure 6. Reaction time (t)-dependent SEC curves of macromonomer≡−S−S−(PS−Br)2 during the ATRP process. Reaction conditions:[styrene]/[initiator]/[CuBr]/[PMDETA] = 150/1/1/1; T = 90 °C;bulk.

Figure 7. IR spectra of (A) ≡−S−S−(PS−Br)2, (B) ≡−S−S−(PS−N3)2, and (C) HB-(S−S−PS)n.

Figure 8. (A) Comparison of two typical SEC curves of HB-(S−S−PS)n monitored by two different detectors: RI (solid line) and MALLS(circle symbol) and (B) reaction time (t)-dependent SEC-RI curves ofHB-(S−S−PS)n, where T = 35 °C and Cpolymer = 0.30 g/mL.



because the scattered light intensity in LLS is proportional tothe product of the weight-average molar mass and the weightconcentration, while the signal from RI is only proportional tothe weight concentration. Figure 8B shows the reaction time-dependent SEC curves of HB-(S−S−PS)n during the self-polycondensation. As the reaction proceeds, the macro-monomer peak becomes smaller while the hyperbranchedpolymer peaks emerged after a few hours. Gradually, DPw andDPn increase to ∼28 and ∼6, respectively, and its polydispersityindex (Mw/Mn) changes from 1.14 to 4.90 (Figure 9) when

most of the initial macromonomers are coupled together,reflecting in a sharp decrease of the NNN stretchingabsorption near ∼2100 cm−1 (Figure 7C). The self-polycondensation nearly ceases after ∼14 h, similar to whatwe observed in a previous study.13 The related polycondensa-tion kinetics analysis can be found elsewhere.13

As discussed before, each branching point of HB-(S−S−PS)ncontains just one disulfide bond that is cleavable when areducing agent is added. The cleavage of each disulfide bondresults in two smaller fragemented hyperbranched chains with athiol end group. Using a combination of static and dynamicLLS, we studied such a degradation kinetics of a fractionatedHB-(S−S−PS)n with a DPw of 35 and a Mw/Mn of 1.40. Thereare different choices of reducing reagents, including thiols,20

phosphines,21,22 and zinc dust.23 DTT was selected in thecurrent study because (1) its induced disulfide reduction is veryefficient owing to its high conformational propensity to form asix-member ring with an internal disulfide bond and (2) it issoluble in a range of organic solvents that can also dissolvepolystyrene easily.28

Figures 10 and 11 show how HB-(S−S−PS)35 chains changeafter the 24 h DTT induced reduction in DMF at 25 °C. Asshown in Figure 10, the elution peak located at ∼24 mLcompletely disappears after the degradation, and only themacromonomer peak (maybe contains few dimers) remains,indicating that the DTT induced degradation of the initial HB-(S−S−PS)35 chains is nearly 100%. The residual dimers areattributed to the limited degradation time (24 h). Such chaindegradation is also directly reflected in the evolution of thehydrodynamic radius distribution [f(Rh)], as shown in Figure11; i.e., the peak position shifts from ∼19 to ∼3 nm afterdegradation. However, it should be noted that the degradationproducts are similar to the initial seesaw-type linear macro-monomers except different end groups, as shown in Scheme 3.Figure 12 shows how the DPw, ⟨Rh⟩, and average chain

density (⟨ρ⟩) of HB-(S−S−PS)35 change during the degrada-

tion at different molar ratios of [DTT]/[−S−S−]; for theconvenience of discussion, DPw,t is used to represent the valueof DPw at time t. As expected, DPw,t and ⟨Rh⟩ gradually decreaseas the degradation (fragmentation) proceeds for a given[DTT]/[−S−S−], and the degradation rate becomes higherwhen more DTT is added. We tested whether all the disulfidebonds can be completely degraded in the range of [DTT]/[−S−S−] = 3−180 and found that most of the initial disulfidebonds were cleaved within 5 h even at [DTT]/[−S−S−] = 3.Figure 12C shows that the average chain density (⟨ρ⟩), definedas Mw/[NA(4/3)π⟨Rh⟩

3], increases with the degradation time,where Mw, ⟨Rh⟩, and NA are the weight-average molar mass, theaverage hydrodynamic radius of hyperbranched chains, andAvogadro’s number, respectively, revealing that larger branchedchains have a lower chain density than its degraded productsand the initial linear macromonomer has the highest ⟨ρ⟩.To analyze the results in Figure 12, and considering the

nature of thiol−disulfide exchanging reaction, we first assumedthat the degradation follows the second-order kinetics, i.e.

−− − −

= − − −t

kd[ S S ]

d[ S S ] [DTT]t t t (2)

where [−S−S−]t and [DTT]t are the molar concentrations ofdisulfide bond and DTT at time t, respectively, and k is areaction rate constant. At t = 0, [−S−S−]t = [−S−S−]0, and att = ∞, [−S−S−]t = 0. Since DTT is excessive, we can treat[DTT]t = [DTT]0 as a constant in eq 2 so that we can rewriteit as a pseudo-first-order equation

− − −− − −

= −[ S S ][ S S ]

et k0

[DTT] t0

(3)

Figure 9. Reaction time (t) dependence of polydispersity index (Mw/Mn), number (DPn), and weight-average degree of polycondensation(DPw) of HB-(S−S−PS)n, where T = 35 °C and Cpolymer = 0.30 g/mL.

Figure 10. SEC-RI curves of HB-(S−S−PS)35 chains before and after24 h DTT-induced reduction in DMF at 25 °C, where [DTT]/[−S−S−] = 18.

Figure 11. Hydrodynamic radius distributions [f(Rh)] of HB-(S−S−PS)35 chains before and after 24 h DTT-induced reduction in DMF at25 °C, where [DTT]/[−S−S−] = 18.



On the other hand, it is well-known that polystyrene isinsoluble in diols so that most of DTT are excluded from theinterior of HB-(S−S−PS)n. Therefore, those external disulfidebonds would be cleaved first. As shown in Scheme 3, thecleavage of each external disulfide bond leads to a decrease ofDPw by one. In this way, we are able to rewrite [−S−S−]t/[−S−S−]0 as DPw,t/DPw,0 so that eq 3 can be rewritten as

= −DP DP etKt

w, w,0 (4)

where DPw,t and DPw,0 are the degrees of polycondensation ofhyperbranched chains at t = t and t = 0, respectively, and K =k[DTT]0.However, Figure 13A shows that the single-exponential

fitting using eq 3 is not satisfactory, which forces us to includethe degradation of those disulfide bonds inside the hyper-branched polystyrene chain. It is expected that the degradationof those internal disulfide bonds would be much slower due to(1) the lower concentration of DTT inside the polystyrene coreand (2) the steric effect of the polystyrene segments aroundeach internal disulfide bond. Empirically, we found that addinga slow degradation process into eq 4 enables us to fit all thecurves in Figure 13B by using a double-exponential fitting asfollows:

= +− −A ADP

DPe et K Kw,

w,0fast slow

t tfast slow

(5)

where A and K are two constants, representing the relativecontributions and rate constants of the slow and fast

degradation reactions, respectively; Afast + Aslow = 1.0. Thereasonable double-exponential fitting actually reveals that thedegradation of disulfide bonds on the periphery of thehyperbranched chain is much faster than those inside.Moreover, Figure 14A shows that the fast degradation

contributes twice more than the slow one; namely, ∼2/3 of thedisulfide bonds are on the periphery. It is interesting to findthat the contributions of the fast and slow degradations areindependent of the DTT concentration, presumably because

Figure 12. Degradation time (t) dependence of (A) weight-averagedegree of polycondensation (DPw,t), (B) average hydrodynamic radius(⟨Rh⟩), and (C) average chain density (⟨ρ⟩) of HB-(S−S−PS)35 inDMF at 25 °C.

Figure 13. DTT-induced degradation time (t) dependence ofnormalized weight-average degree of polycondensation (DPw,t/DPw,0) of HB-(S−S−PS)35 in DMF at 25 °C, where dashed andsolid lines represent a single-exponential fitting (eq 4) and a double-exponential fitting (eq 5), respectively.

Figure 14. DTT initial concentration ([DTT]0) dependence of (A)relative contributions (A) of fast and slow degradation processes and(B) reaction rate constants kfast and kslow calculated from fitting of datain Figure 13 on the basis of eq 5.



the distribution of the disulfide bonds inside a hyperbranchedchain is not influenced by the presence of DTT. In addition,Figure 14B reveals that both Kfast and Kslow increase linearlywith the DTT concentration, i.e., Kfast = kfast[DTT]0 and Kslow =kslow[DTT]0, indicating that the cleavage of the disulfide bondon a polymer chain by DTT is very similar to those smallmolecule thiol−disulfide exchange reactions.53−55 It is worthnoting that both kfast and kslow are much smaller than theapparent rate constants (105−106 min−1 M−1) of smallmolecule thiol−disulfide exchange reactions in DMF.56Finally, we studied the effect of solvent on the degradation

kinetics of HB-(S−S−PS)35 because it is practically importantto choose a proper solvent to degrade a given polymer chain.Figure 15 shows that the degradation rate increases with the

solvent polarity. To our knowledge, this is the first study of howthe solvent polarity affects the degradation kinetics of disulfide-functionalized hyperbranched polymers. Quatitatively, the fastand slow degradation rate constants in DMF are ∼2 and ∼5times higher than those in THF and DCM, respectively. It hasbeen known that the reducing power of DTT depends on howeasily the thiol groups of DTT are ionized into the thiolateform −S− because only a free thiolate can attack the disulfidelinkage on a chain.28,52 Generally, organic solvents with a higherpolarity are able to polarize the thiol group to produce morefree thiolate.

■ CONCLUSIONUsing the interchain “clicking” of disulfide-functional macro-monomer ≡−S−S−(PS−N3)2, we have successfully preparedmodel hyperbranched polystyrene chains with both uniformsubchains and controlled locations of cleavable disulfidelinkage. The DTT-induced degradation of such modelhyperbranched chains in DMF contains a fast and a slowprocess because the degradation of the disulfide bonds on thechain periphery is much faster than those inside, attributing to ahigher DTT concentration outside and a weaker steric effect ofthe polystyrene segments. Not only the category of monomerand cleavable linkage but also the coupling method developedin the current study can be alternated to make variousstructure-defined degradable hyperbranched polymer chains,which makes further studies of the correlation betweenmicroscopic structures and macroscopic properties of degrad-able hyperbranched polymers possible. We can envision thatsuch degradable hyperbranched chains can be used to

encapsulate active chemicals and release them in a controllablefashion because hyperbranched chains have a smaller size and aless tendency to aggregate with each other than their linearcounterparts for a given molar mass.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail [email protected] (L.L.).*E-mail [email protected] (X.Y.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe financial support of the Ministry of Science andTechnology of China Key Project (2012CB933800), theNational Natural Scientific Foundation of China Projects(20934005, 51173177 and 21274140), and the Hong KongSpecial Administration Region Earmarked Projects(CUHK4036/11P, 2130281/2060431; CUHK4035/12P,2130306/4053005; and CUHK7/CRF/12G, 2390062) isgratefully acknowledged.

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Figure 15. DTT-induced degradation time (t) dependence ofnormalized weight-average degree of polycondensation (DPw,t/DPw,0) of HB-(S−S−PS)35 in three different solvents at 25 °C,where lines represent double-exponential fittings of eq 5.



mailto:[email protected]:[email protected]

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